fix openjdk-src bootimage build

The main change here is to use a lazily-populated vector to associate
runtime data with classes instead of referencing them directly from
the class which requires updating immutable references in the heap
image.  The other changes employ other strategies to avoid trying to
update immutable references.

classpath/java/lang/Class.java

@@ -572,10 +572,6 @@ public final class Class <T> implements Type, AnnotatedElement {
     return (T) o;
   }
 
-  public Object[] getSigners() {
-    return vmClass.addendum.signers;
-  }
-
   public Package getPackage() {
     if ((vmClass.vmFlags & PrimitiveFlag) != 0 || isArray()) {
       return null;